Black-blood MRI at 7T using 2D RARE MRI: In vitro testing and in vivo demonstration

Vessel walls play a crucial role in many inflammatory vascular diseases. Vessel wall imaging (VWI) using mangnetic resonance imaging (MRI) is one of the few methods by which vessel walls and inflammation can be visualized noninvasively, in vivo, and without ionizing radiation. VWI is based on black-blood (BB) MRI, where the signal from flowing blood is suppressed and contrast agent accumulation in the (inflamed) vessel wall is highlighted. Here, high resolution, T1 weighting, suppression of fat and flowing spins is essential. Whereas VWI is often applied in humans, only very few reports describe its use in small animals. Here, we investigated whether BB MRI for rodents can be implemented using a state-of-the-art, but commercially available, preclinical MRI system and imaging sequence. We identified 2D spin-echo (RARE)-based BB-MRI as a promising sequence that is widely available and not vendor dependent. First, we investigated the properties of the sequence in vitro with respect to image contrast, resolution, the suppression of signal of flowing spins and fat using a newly developed, 3D-printed model setup (cylindrical model with exchangeable nuclear magnetic resonance tubes and flow tube in agarose, printed with stereolithography). For example, good signal-to-noise ratio, BB and T1 contrast were obtained for TE = 5 ms for slice thickness equal or below 0.352 mm or slice thickness = 0.8 mm with TE at least 25 ms. In vivo, we obtained a pronounced BB effect for both intracranial and abdominal vessels of healthy rats down to a 0.25 mm diameter in no more than 1:36 min with TE = 12 ms, TR = 750 ms, voxel 156 × 156 × 800 µm³, and 11 slices. Compared to in vitro, we were able to reduce TE without apparent artifacts likely because the flow was faster in vivo than in vitro. Additionally, we needed to increase the resolution to image small vessels. Thus, we found that BB-MRI with 2D spin-echo sequences is feasible on rodents with state-of-the-art, commercially available preclinical MRI systems. We believe that these results will facilitate the development and application of rodent VWI in longitudinal studies, which, in comparison to histology, may reduce the number of needed animals and intersubject variability at the same time.